1. Field of the Invention
This invention relates generally to AC coupled operational amplifier circuits and more particularly to AC coupled operational amplifiers providing a precise gain and a low frequency stabilizing pole.
2. Description of the Prior Art
Typically, integrated precise inverting AC coupled operational amplifiers are limited in ability to implement stabilizing low frequency poles. Since an AC coupled operational amplifier must have DC feedback in order to be stable, it is desirable to have the lowest stabilizing low frequency pole which is possible to obtain. A low frequency pole also allows an AC coupled operational amplifier to have unity gain at DC. To implement a low frequency pole, a large load resistance of several meg-ohms is required, generally necessitating the use of components external to the integrated circuit. The use of a switched capacitor to simulate a resistor is well known in the art of integrated operational amplifier circuits. However, when switched capacitor techniques are used, the clock frequencies of the switched capacitors generally must be at least ten times greater than the highest pass band frequency of the circuit to prevent errors such as aliasing and sampling distortion. Therefore the clock frequencies must be as much as one thousand times greater than the lowest pole frequency of the circuit. The simulation of a resistance in the tens of meg-ohm range at clock frequencies in the hundreds of kilo-hertz range requires a switched capacitance on the order of one thousandth of a pico-farad if the other capacitors of the circuit are to be kept below a practical value of ten pico-farads. Since the best capacitors which are currently manufactured have an inherent parastic element which is much greater than one thousandth of a pico-farad, it is not possible to simulate a resistance in the tens of meg-ohm range with a switched capacitor.